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Survey Sample Size

Survey Sample Size. MKTG 3342 Fall 2008 Professor Edward Fox. Sample Size Determination. Convenience – Say … about 100. Rule of Thumb - At least 30 per each subgroup (e.g., males/females) that will be analyzed.

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Survey Sample Size

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  1. Survey Sample Size MKTG 3342 Fall 2008 Professor Edward Fox

  2. Sample Size Determination • Convenience – Say … about 100. • Rule of Thumb - At least 30 per each subgroup (e.g., males/females) that will be analyzed. • Budget Constraint - Have a $300 budget for sampling. On average it costs $2 per returned questionnaire. Then go for sample size of 150.

  3. Number of Subgroup Analyses Regional or Special Regional or Special National National • Comparable Studies or Industry Average Typical Sample Sizes for Studies of Human and Institutional Populations People or Households Institutions 500-1500 1500-2500 2500+ 200-500 500-1000 1000+ 50-200 200-500 500+ None or few Average Many 200-500 500-1000 1000+

  4. Determining Sample Size Using Statistical Methods There are statistical formulas for computing sample sizes. These consider three aspects: Precision: Percent of sampling error deemed acceptable by the researcher Confidence: How confident is the researcher that the true average value lies in the interval [lower, upper] estimated Variance: Dispersion of the true value across the population

  5. Determining Sample Size • In general: • If you desire greater precision in your estimate, you need a larger sample size, other things being equal • If you want greater confidence in your estimate, you need a larger sample size • If the estimated variance in the population is high, then you need a larger sample size

  6. Determining Sample Size from Web There are many sample size calculators on the web. Two such websites are: http://www.steinermarketing.com/calc_sample_size.htm http://www.dssresearch.com/toolkit/sscalc/size.asp The formula on the first website is for estimating proportions. Use 95% and 99% confidence level, and confidence interval (allowable error) to be 2% to 10%. Use any population size such as 100, 1000, 10,000. See what happens to sample size as you vary the parameters.

  7. What About Response Rates? • The calculated sample size is the number of desired actual responses, or completed questionnaires • In the real-world not all surveys sent out are completed (response rates are less than 100%) • You must incorporate the expected response rate when deciding how many questionnaires to send out or how many people to call

  8. Response Rate Calculation For Mail Surveys: Response # Usable Surveys Returned = # Surveys # Surveys Returned Rate - Mailed " Not Deliverabl e"

  9. Mail Surveys Estimating the number of surveys required to achieve given sample size: n Surveys Required = [(1-U) RR] n = required sample size U= estimated proportion “not deliverable” RR = estimated response rate (proportion)

  10. Mail Survey Example • You have determined that sample size of 200 will allow reasonable precision and confidence for your estimates of important population parameters. You will be conducting a mail survey of households in Highland Park. You expect that about 5% of mail will be undeliverable and the expected response rate is 10%. How many mail questionnaires should you send out?

  11. Response Rate Calculation For Telephone Surveys: #Completed Interviews RR = #Completed + #Refusals + #No Answers Interviews

  12. Telephone Survey Estimating the number of calls required to achieve given sample size: n Total Calls = [(1-NE) (1-R) (1-NA)] where: n = required sample size NE= estimated proportion of non-eligibles R = estimated proportion of refusals NA = estimated proportion of no answers

  13. Telephone Survey Example • You have determined that a sample size of 200 will allow reasonable precision and confidence for your estimates of important population parameters. You will be conducting a telephone survey of university students ages 20 and older. After checking with university registration officials you know that 50% of all university students meet this criterion. Further, you expect about 20% of the people you contact not to participate in the survey and about 15% not to be reachable even after trying at several different times on different days of the week. How may total calls should you expect to make for this project?

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